Photoelectron microscopes are used for understanding the surface state of material systems. A photoelectron microscope uses photons to excite the emission of electrons from the surface of a material into a vacuum where spatial variations in the electron flux is used to image the surface. The information that can be extracted from an image includes variations in the chemistry, the crystal structure, the position of the Fermi level, and the surface potential. The kinetic energy of the emitted electron, Ekinetic, is related to the energy of the photon, Ephoton, by the relationship:Ebinding=Ephoton−Ekinetic−Φ
Where Ebinding is the binding energy of the electron in the material and Φ is the work function. If the photon has sufficient energy it can cause electron emission into the vacuum from either a localized atomic level, a valency band, or a conduction level state in the material. Depending on the energy of the incident photon which can excite a range of different electronic transitions in the atoms and valency bands of the material the resultant kinetic energy of the electron can provide detailed information about the atomic species and the chemical state of the material surface. When X-ray energies are used for excitation of core level electrons the techniques is known as X-ray photoelectron spectroscopy. The emitted energies in XPS are typically below 1.5 keV. Because of the chemical species and chemical state specificity the technique is also known as electron spectroscopy for chemical analysis or ESCA. The electrons leave the surface with a range of energies depending on their individual history and losses in the surface of the solid. The energy of the photoelectrons leaving the sample is determined using an electron energy analyzer, usually a high resolution concentric hemispherical analyzer (CHA). Sweeping the analyzer with energy gives a spectrum with a series of photoelectron peaks. Because the range of electrons with energies below 1.5 keV in a material is very small the spectrum represents the chemistry of the top few atomic layers of a material. Contrast in the spatial emission of electrons from different areas of the material are thus due to differences in elemental species and their chemical states across the surface. The contrast is present as both total intensity of the emitted electrons and as structure in the electron energy distribution. This structure can be imaged by either changing the photon energy to excite a particular core level or by analyzing the kinetic energy of the emitted electrons and imaging only those electrons in a range of interest.
In an XPS-microscope an electron energy analyzer is used. Ideally, the energy analyzer must preserve the image with a minimum of distortion and loss of spatial resolution. There have been two types of analyzer used for this task, a high pass analyzer and a band pass analyzer. The band pass analyzer is typically a retarding grid arrangement which rejects electrons energies below the grid retarding potential. The image is formed by differentiating the electron signal intensity as the energy is swept through a feature of interest in the spectrum. Although this is a straightforward technique the problem is that it is intrinsically noisy and of low energy resolution. Consequently, there has been a trend to using a band pass analyzer such as a CHA which separates out and images only those electrons in the range of interest. However, there is a problem with this approach. It is difficult to transfer the electrons from the imaging lens system into the energy analysis system and preserve the image. The reason for this problem is the type of imaging lens that is used in XPS-microscopy is typically a magnetic immersion lens. An immersion lens is used to collect as much of the available emitted electrons as is then possible to reduce the time required to collect images. One of the most successful approaches to this is a development of the work of Beamson et. al., Nature Vol. 290, p. 556, 1981 and Turner U.S. Pat. No. 4,486,659. Beamson et. al. and Turner teach that an axially symmetric divergent magnetic field can generate an enlarged image of a photo-emissive surface while preserving the original energy distribution. They further teach that an energy resolved image can be made by inserting a retarding field electron energy analyzer into the diverging magnetic field. This work was later developed by several authors with several variations in instrument design. Kim et. al. Review of Scientific Instruments, vol. 66(5) p. 3159, 1995 teach that the field of a magnetic projection microscope can be terminated by a stack of magnetic grids in front of the entrance aperture of a band pass spectrometer. However, it is not clear that the terminating field has an influence on the spatial resolution which was measured at 3.5 micron for 4 eV photoelectrons. Hirose U.S. Pat. No. 5,045,696 teaches that by reducing the energy of the collected photoelectrons to those emitted with energies below 1 eV the spatial resolution can be improved in a photoelectron microscope using a projection lens. Hirose used a pulsed X-ray source and a pulsed electronic gating method to only image slower electrons but this necessarily means the high energy electrons that have chemical specificity cannot be imaged. Sekine et. al. U.S. Pat. No. 5,285,066 have an alternative imaging lens that does not require a strong magnetic field at the sample and thus they can image more easily into a band pass analyzer. They use a concentric hemispherical analyzer CHA with a long entrance slit. A line of the image is thus imaged and energy resolved and the sample areas is swept electronically to produce a two dimensional energy resolved image. Walker U.S. Pat. No. 4,810,879 teaches that a magnetic immersion lens can be used to focus photoelectrons into a spectrometer. The immersion lens of Walker has a limited magnetic field extent and is used for focusing into subsequent electron lenses and is not used for projection of the electrons. The variations of XPS microscopes are described above are included in a variety of commercially available systems. However, none of these systems has a spatial resolution significantly below 5 microns. Further, the data from the instruments is either an image at a particular energy or a energy spectrum, not both. Further, for electrons above a few electron volts the spatial resolution is much greater than 5 microns.
The high energy photoelectrons containing much of the chemically specific information that includes both elemental and chemical state are difficult to image at a high spatial resolution.
When UV photons are used the spatial resolution of the microscope is much improved as the photoelectrons are now emitted with a lower energy and the cyclotron radius is smaller. However, even at the strongest continuous magnetic fields available, 10-20 T (Tesla), the spatial resolution of the Fermi level is still only in the 1-2 micron region.
Much of the phenomena associated with materials science and electronics is at a much smaller scale than currently obtainable with X-ray microscopes with much interest in the 100 nm range and below.
What is desired, therefore, is a photoelectron microscope that is suitable for imaging surfaces with high spatial and energy resolution. The information contained in the images includes chemical species, chemical state, Fermi level, and surface potential.